1. Field of the Invention
The present invention relates to a liquid crystal display device in which a driving circuit for supplying driving signals to at least one of the opposing electrodes is installed on at least one substrate of liquid crystal display cells.
2. Description of the Related Art
A matrix-type liquid crystal display device for displaying television pictures comprises, an electrode for display formed on a pair of opposed glass substrates, and a liquid crystal layer sandwiched between a pair of substrates. The liquid crystal display device includes the simple matrix type and the TFT (thin film transistor) active matrix type. For the simple-matrix-type liquid-crystal display device, many striped scanning electrodes are arranged in parallel on one glass substrate and many striped signal electrodes orthogonal to the scanning electrodes are arranged in parallel on the other glass substrate.
For the TFT-active-matrix-type liquid-crystal display device, many pixel electrodes and thin film transistors (TFTs) for selecting each pixel electrode are arranged in rows and columns on one glass substrate and opposing electrodes are formed on the other glass substrate.
Recently, one of the matrix-type liquid-crystal display devices has been proposed in which the display driving circuit is installed on the terminal arranged portion of the glass substrate.
FIGS. 1 and 2 show a simple-matrix-type liquid-crystal display device having the existing display driving circuit.
In FIGS. 1 and 2, a pair of glass substrates 1 and 2 faced at the both sides of a liquid crystal layer LC are bonded through the frame-shaped sealing member 3 enclosing the liquid crystal sealing area. Many striped scanning electrodes 4 (transparent electrodes) are arranged in parallel on the glass substrate 1 (upper substrate in the drawing) of the pair of glass substrates 1 and 2. Many striped signal electrodes 5 (transparent electrodes) orthogonal to the scanning electrodes 4 are arranged in parallel on the other glass substrate 2 (lower substrate in the drawing). In addition, orientation films 6a and 6b are formed on the surface where electrodes of the both substrates 1 and 2 are formed. For the liquid crystal display device, the signal electrodes 5 are separated at the middle in the longitudinal direction to individually drive each signal electrode 5 so that the display driving duty ratio will be decreased.
Several (two in the drawing) scanning driving circuit elements 7a are installed on one end of the glass substrate 1. Several (four in the drawing) signal driving circuit elements 7b are installed on both ends of the other glass substrate 2 respectively. The output terminal of each scanning driving circuit element 7a is installed by solder or the like on the end of a driving circuit connecting lead wire 4a which is extended from the end of each scanning electrode 4 and arranged on the surface of the glass substrate 1. The output terminal of each signal driving circuit element 7b is installed by solder or the like on the end of a driving circuit connecting lead wire 5a which is extended from the end of each signal electrode 5 and arranged on the surface of the glass substrate 2. Numeral 8a is an external-circuit connecting wire arranged on the surface of the glass substrate 1 and numeral 8b is an external-circuit connecting wire arranged on the surface of the glass substrate 2. The input terminals of the driving circuit elements 7a and 7b are connected by solder or the like with the external-circuit connecting wires 8a and 8b respectively. The driving circuit elements 7a and 7b use an IC chip in which many MOS-type transistors are formed on a single-crystal silicon substrate. The scanning and signal display driving circuits comprise several driving circuit elements (IC chip) 7a and 7b respectively. The existing active-matrix-type liquid-crystal display device includes two types: one is the type using a single crystal silicon substrate as each of the pair of substrates of the liquid crystal cell on which single-crystal MOS-type transistors made of single crystal silicon are formed as the selecting transistor for selecting a pixel electrode, and the other is the TFT active-matrix-type using thin film transistors (TFTs) as the selecting transistor for selecting a pixel electrode.
The active-matrix-type liquid-crystal display device using the single-crystal MOS-type transistor uses a silicon substrate made of single crystal silicon as either substrate on which the MOS-type selecting transistors are formed. Each pixel electrode of the active-matrix-type liquid-crystal display device is connected to the source electrode of each selecting transistor on the silicon substrate. The gate electrode and drain electrode of each selecting transistor are connected to many scanning lines and data lines which are orthogonally formed on the silicon substrate.
For the TFT active-matrix-type liquid-crystal display device using thin film transistors, many scanning lines and many data lines orthogonal to the scanning lines are formed on a glass substrate. A gate electrode, gate insulting film, semiconductor layer comprising amorphous silicon, and selecting transistor electrodes are formed at each intersection of the scanning and data lines. The gate electrode and drain electrode of each selecting thin-film transistor are connected to the scanning line and data line respectively, and the source electrode is connected to the pixel electrode formed on the glass substrate.
For every liquid-crystal display device shown above, opposing electrodes are formed on the opposing glass substrates.
For the active-matrix-type liquid-crystal display device using a single-crystal substrate as the one of the pair of substrates of liquid crystal cell, the display driving circuit comprises many single-crystal MOS-type transistors for a driver formed on the periphery of a line lead-out portion on the silicon substrate.
For the TFT active-matrix-type liquid-crystal display device, the scanning and signal display driving circuits are composed of several driving circuit elements consisting of IC chips respectively and each driving circuit element is installed on the periphery of either glass substrate forming the pixel electrodes and the thin film transistors for selectively driving them. For the TFT active-matrix-type liquid-crystal display device the terminal of each scanning driving circuit element is connected to the lead wire extended from the scanning line (gate line) connecting with the gate electrode of the thin film transistor for selectively driving pixel electrodes. The terminal of each signal driving circuit element is connected to the lead wire extended from the data line connecting with the drain electrode of the thin film transistor.
However, because the display driving circuit of the above existing liquid-crystal display device using IC chips is configured by installing several driving circuit elements consisting of IC chips on the glass substrate where display electrodes are formed, each of the above driving circuit elements must be connected to each lead wire arranged on the glass substrate. Therefore, it is complicated to manufacture the liquid-crystal display device. For the active-matrix-type liquid-crystal display device using a single crystal substrate, the operation speed of the display driving circuit is high because the transistors for selecting pixel electrodes and for the driver use the single-crystal MOS-type transistor. However, the liquid-crystal display device manufacturing cost is very high because high-purity single-crystal silicon substrate should be used for the substrate. Moreover, there is a problem that a large screen can hardly be made because it is difficult to manufacture single-crystal silicon substrate with a large area.
Meanwhile, the TFT active-matrix-type liquid-crystal display device can use inexpensive and large-area glass substrates because the transistors for selecting pixel electrodes and for a driver use a thin film transistor. Therefore, the liquid-crystal display device manufacturing cost can greatly be decreased. Moreover, it is possible to manufacture a large screen.
However, if the display driving circuit of the TFT active-matrix-type liquid-crystal display device is configured with thin film transistors using amorphous silicon as a semiconductor, high-speed display driving cannot be made because the operation speed of the display driving circuit decreases.
Therefore, for the TFT active-matrix-type liquid-crystal display device, it is preferable to use the thin film transistor using polycrystal silicon for the semiconductor layer. Because the thin film transistor using polycrystal silicon for the semiconductor layer has a higher operation speed than the transistor using amorphous silicon, the operation speed of the display driving circuit can be improved.
For the thin film transistor using the polycrystal silicon for the semiconductor layer, however, heat treatment should be executed to form the polycrystal silicon semiconductor layer by heating a deposited layer of amorphous silicon at a high temperature. Therefore, the substrate should be a heat-resistant substrate capable of standing the high temperature. The existing TFT active-matrix-type liquid-crystal display device comprises thin film transistors for a driver in which the display driving circuit is made at the end of the line lead-out portion of either glass substrate where the pixel electrode selecting thin-film transistors and pixel electrodes are formed. Therefore, to use the thin film transistor for a driver using polycrystal silicon for the semiconductor layer, the above glass substrate must be made of heat-resistant glass such as quartz glass. Because the heat-resistant glass is much more expensive compared with normal glass (e.g, soda-lime glass coated thereon with silicon dioxide film), the liquid-crystal display device manufacturing cost increases when the heat-resistant glass is used for the above glass substrate.
The present invention is made in view of the above situation and it is an object of the present invention to provide liquid-crystal display devices which have display driving circuits and can easily be manufactured at a low cost.